Duplex signalling circuits are used extensively between central telephone offices or between a central office and a subscriber station to send and receive electrical signals representing that certain events have occurred at the location. Such an event, for example, would be the detection by the central office of a change in the on-hook or off-hook status of a telephone set at a subscriber station or the reproduction of dialed digits.
Conventional systems of this type use relays and transistors to transmit and receive bilevel D.C. signals between two different network locations which are connected together by a trunk or branch telephone line comprised of two conductors. In a duplex signalling network, the same pair of conductors are used to transmit and to receive signalling information such as supervisory signals between the two locations. Because of the presence of stray leakage currents, the ground potential at one location may be different from that at the other location. Therefore, these systems utilize passive voltage divider circuits formed by resistors of known value to establish an initial bias or reference voltage common to the two locations. The voltage divider circuits which are used for this purpose are typically constituted by two resistors of about 600 ohms and 400 ohms, respectively, connected in a series circuit with one end connected to ground potential and the other end connected to a source of fixed negative potential of typically -48 volts. The negative voltage of about -20 volts appearing at the point in the circuit between the two resistors is applied to one conductor to provide the requisite D.C. potential to the trunk or branch line for proper biasing.
One disadvantage of using a passive voltage divider in these types of duplex systems is that the divider constantly dissipates about 2.5 watts of electrical power. Moreover, the fixed negative voltage applied to the one line by the divider may act in opposition to the bilevel D.C. signalling voltage applied to the other line by a D.C. transmitter to indicate a change in state of a condition in the network at one location. The magnitude of the step D.C. signalling voltage which is available to transmit the information on the other line may be bucked by the reference voltage on the one conductor or line generated by the voltage divider. As will be apparent, this reduces the level of the signalling voltage between the two lines and consequently, limits the distances between the signal transmitting and the signal receiving locations.
In conventional duplex networks, the transmitted signal is detected by a differential relay or electronic comparator coupled across the two conductors of the line and a line balancing network for balancing the line impedance. The differential relay is responsive to currents which are generated by a transmitter at location A at one end of the line which includes a voltage divider circuit, and the current components flowing in the line produced by a transmitter at location B at the opposite end of the line. By suitable adjustment of the balancing network, the flux generated in a receive relay at location A by the current component generated by the location A transmitter is cancelled out by subtraction. On the other hand, the current component flowing in the line generated by the transmitter at location B will aid in generating a flux in the receive relay coils at location A. The polarity of the flux produced depends upon the status of the location B transmitter. The armature of the receive relay at location A will be attracted or repelled depending upon the polarity of the total flux in its coils to change the states of contacts under the control of the relay armature. The electronic comparator essentially performs the same function as the relay, but is responsive to voltage drops across its inputs. The output of the comparator is a voltage level which may be used to drive or control other equipment in the same manner that the relay contacts will perform such functions. Since the relay or comparator input is differential in nature, it has the inherent capability of rejecting common-mode signals. Common-mode signals take the form of two current components of equal magnitude flowing through both line conductors in the same direction. These currents have no effect on the state of the relay or electronic comparator because they have equal cancelling effects on their inputs. Common-mode signals are often referred to by those in the art as "longitudinal signals" to distinguish them from so-called "transversal signals". Transversal signals are the voltage differences between the two respective line conductors, commonly referred to by those in the telephone art as the "tip" and "ring" conductors. The constant voltage generated by the voltage divider circuit appears as a common-mode signal and therefore, diminishes the available margin for rejecting additional common-mode signals induced by other sources, such as power line electromagnetic fields.